This invention relates to flexible polyurethane foams used in carpet cushions and flooring underlayments. Produced at or preferably above atmospheric conditions from certain foaming mixtures, the foams of this invention provide higher support and higher durability than more conventional polyurethane foams previously used in these applications.
Polyurethane foams with varying density and hardness may be formed. Hardness is typically measured as IFD (xe2x80x9cindentation force deflectionxe2x80x9d) or CFD (xe2x80x9ccompression force deflectionxe2x80x9d). Specifically, IFD25 is the force required to compress the foam to 25% of its original thickness or height, and IFD65 is the force required to compress the foam to 65% of its original thickness or height. Tensile strength, tear strength, compression set, air permeability, fatigue resistance, support factor, and energy absorbing characteristics may also be varied, as can many other properties. Specific foam characteristics depend upon the selection of the starting materials, the foaming process and conditions, and sometimes on the subsequent processing.
Among many applications, polyurethane foams are widely used for carpet cushions and underlayments. Typically, a one-half inch thick foam pad is placed between the subflooring and the carpet. For such a thin pad to be an effective cushion, the foam generally must be quite firm, and will typically have an IFD25 above 100 pounds.
Two categories of polyurethane foams presently are used as carpet cushions. xe2x80x9cPrimexe2x80x9d carpet cushions are foams specifically poured for that application. Prime carpet cushions generally have densities of about 3.5 pounds per cubic foot (xe2x80x9cpcfxe2x80x9d or xe2x80x9clb/ft3xe2x80x9d) or less. xe2x80x9cRebondxe2x80x9d carpet cushions are formed from pieces of recycled polyurethane foam. Typically, the recycled polyurethane foams had densities of about 1-2 pcf. The recycled polyurethane foams are ground to smaller pieces that are then mixed together with bonding agents and compressed and cured to form a composite material that has a density generally in the range of 6 to 8 pcf. The composite material is sliced to desired thickness to form the rebond carpet cushion. Because it is formed from recycled foams, rebond tends to have a variegated or irregular surface appearance.
A key performance measurement for carpet cushion is durability. Heretofore, prime carpet cushion with its generally lower density has been perceived as less durable than rebounded carpet cushion. On the other hand, prime carpet cushion benefits from having consistent face surfaces, whereas the lumpy surfaces of rebonded carpet cushion have been perceived to indicate lower quality. In general, consumers subjectively correlate a foam having a fine cell structure and a smoother surface with better comfort.
Typically, xe2x80x9cdurabilityxe2x80x9d of a foam is gauged by its fatigue resistance, which can be measured using two methods: (1) the roll shear test; and (2) the hexapod fatigue test. The roll shear fatigue test was developed to determine the fatigue resistance of foam carpet cushions under simulated end use compression and shear. In this test, a foam sample is cut and formed into a belt that is wrapped around a top roll of a pair of compression rollers. In setting up this test, a 190-pound weight is applied to the sample to determine the amount of compression that the foam cushion undergoes during a normal walking step. This same amount of compression is applied to the foam sample during the roll shear test by adjusting the gap between the two rolls of the test unit. Shear is supplied by rotating the rolls at different rates. Hence, in each cycle, the foam sample belt is sheared and compressed as it is pulled through the rotating rolls. Typically, the roll shear test is run for 12,000 cycles, and fatigue is measured by determining the percent loss of IFD25 and thickness. In calculating the percent loss, the initial IFD25 and thickness measurements are taken before the start of the test, and the final measurements are taken 24 hours after the completion of 12,000 cycles.
Using the roll shear fatigue test, it is expected that 12,000 cycles approximate the wear on a foam carpet cushion after five years of residential use. In general, higher density foams will provide better fatigue resistance. For example, after 12,000 cycles in the roller shear fatigue test, a conventional 3 pound per cubic foot (pcf) prime carpet cushion foam retains about 50% of IFD25, whereas a rebonded foam with a density of 6 to 8 pcf retains about 70% of IFD25.
The second test, the hexapod fatigue test, is described in ASTM D 5252-92. In this test, a rotatable drum containing a metal hexapod with six polyurethane studs is used to measure wear and fatigue. Specifically, one surface of the foam sample to be tested is attached to the inside surface of the rotatable drum and a carpet sample is attached to the opposite surface of the foam specimen. The metal hexapod rolls randomly on the surface of the carpet inside the rotating drum for a specified number of revolutions (cycles). The foam samples are then inspected and rated visually on a scale of 0 to 5. The higher the rating indicates the better quality of the sample, and therefore higher durability. Again, higher density foams will usually perform better in this test.
For carpet cushion applications, polyether polyurethane foams are overwhelmingly preferred over polyester polyurethane foams, because the former have far superior hydrolytic stability and hence will be more resistant to degradation from exposure to moisture. Cellular polyurethane structures typically are prepared by generating a gas during polymerization of a liquid reaction mixture comprised of a polyester or polyether polyol, an isocyanate, a surfactant, catalyst and one or more blowing agents. The gas causes foaming of the reaction mixture to form the cellular structure. The surfactant stabilizes the structure.
Once the foam-forming ingredients are mixed together, it is known that the polyurethane foam may be formed under either elevated or reduced controlled pressure conditions. PCT Published Patent Application WO 93/09934 discloses methods for continuously producing slabs of urethane polymers under controlled pressure conditions. The foam-forming mixture of polyol, isocyanate, blowing agent and other additives is introduced continuously onto a moving conveyor in an enclosure with two sub-chambers. The foaming takes place at controlled pressure. Reaction gases are exhausted from the enclosure as necessary to maintain the desired operating pressure. The two sub-chambers, a saw, and air-tight doors are operated in a manner that allows for continuous production of slabstock polyurethane foam.
U.S. Pat. No. 5,804,113 to Blackwell, et al., shows a method and apparatus for continuously producing slabstock polyurethane foam under controlled pressure conditions in which a layer of gas surrounds the reaction mixture during free rise expansion of the reaction mixture to prevent pressure fluctuations. Blackwell generally describes foam reaction mixtures that may include a variety of polyols and isocyanates, and does not express preference for any specific combinations.
U.S. Pat. No. 4,777,186 to Stang, et al., describes a method of foaming in a pressurized chamber held above atmospheric pressure (i.e., in the range of about 0.5 to 1000 psig). In addition to the gases emitted during foaming, additional gases may be introduced into the chamber to maintain the elevated pressure during foaming. The resulting foams have a higher IFD to density ratio than those previously known in the art.
Fine-celled, high durability polyurethane foams are produced using an isocyanate component with a high methylene diisocyanate (xe2x80x9cMDIxe2x80x9d) content, in U.S. Pat. No. 6,136,878 to Free, et al. The cells were fine with over 87 cells per inch, as measured via pressure drop. Foams from 3.2 to 4.3 lb/ft3 density could be made at atmospheric pressure with the fine cell structure.
To date, the use of MDI in foam formulations for carpet cushions is still limited, primarily due to processing difficulties when foaming MDI foam formulations in commercial foaming machines. Specifically, trough build-up and a steep reactivity profile for MDI-containing polyurethane foams make the operating window very narrow. The operating window is often defined by the range of catalysis that can be used to make an acceptable foam that is free of any physical cracks, cleavage and densification. Therefore, it would be more desirable to produce a fine cell polyurethane carpet cushion using a toluene diisocyanate (xe2x80x9cTDIxe2x80x9d) formulation with a larger operating window. Furthermore, especially for carpet cushion applications, it would be desirable to produce foams with higher durability and higher IFD, yet still maintain a relatively low foam density (i.e., at about 3 to 4 lb/ft3).
The prior art does not disclose methods for making fine cell, high durability polyurethane foams at a moderate density, nor does the prior art disclose carpet cushions made from such foams.
According to the invention, flexible, fine celled, high durability polyurethane carpet cushioning foams are produced by preparing a specified foam reaction mixture and foaming that mixture at or above atmospheric conditions, preferably at pressures in the range of 1.05 to 1.5 bar (absolute), most preferably 1.05 to 1.3 bar (absolute). The reaction mixture contains (a) a polyol mixture of (i) about 10 to 70 percent by weight total polyols of a polyether polyol having up to 85 percent ethylene oxide groups, and !i having a hydroxyl number in the range of about 20 to 60 and a functionality from 2.8 to 3.5, and (ii) about 30 to 90 percent by weight total polyols of a graft polyol having a ratio of styrene to acrylonitrile of about 70/30 to about 50/50, and having a hydroxyl number in the range of about 25 to 60 and a functionality from 2.5 to 3.0; (b) toluene diisocyanate wherein the isocyanate index is in the range of 95 to 135; and (c) from about 1.2 to 2.5 parts per hundred parts polyol of water as a blowing agent.
Most preferably, the foam-forming composition contains up to 2 parts per hundred parts polyol of an amine catalyst, up to 2 parts per hundred parts polyol of a surfactant, up to 0.5 parts per hundred parts polyol of an organotin catalyst, and up to 2 to 6 parts per hundred parts polyol of a cross linking agent.
In addition, excellent results have been obtained using a preferred polyol combination of (a) from 25 to 60% by weight total polyols of polyether polyol (functionality 3.1 to 3.3), having 50 to 80 percent EO groups and a hydroxyl number in the range of 25 to 35, and (b) from about 40 to 75 percent by weight total polyols of a graft polyol having a ratio of styrene to acrylonitrile of about 70 to 30, and having a hydroxyl number in the range of about 25 to 30 and a functionality from 2.8 to 2.9, and reacting with toluene diisocyanate wherein the isocyanate index was in the range of 105 to 115. In this preferred embodiment, from about 1.6 to 2.2 parts per hundred parts polyol of water as a blowing agent; and up to 1.0 parts per hundred parts polyol of a surfactant are included in the reaction mixture.
The resulting polyurethane foams have densities in the range of about 2.5 to 4.5 pounds per cubic foot, preferably about 3 to 4 pounds per cubic foot, and fine cell sizes of about 70 pores per linear inch or finer (e.g., xe2x89xa770 ppi), most preferably about 80 pores per linear inch or finer (e.g., xe2x89xa780 ppi). Such foams further retain over 70% of their IFD25 after 12,000 cycles and over 60% after 50,000 cycles in the roller shear fatigue test. In addition, the foams have a low surface roughness (preferably Ra under 0.25 mm as measured by perthometer). The foams are well suited for use as prime carpet cushion or as component foam for rebonded carpet cushion.